Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a permanent record of the charge image.
Various types of photoconductive insulating elements are known for use in electrophotographic imaging processes. In many conventional elements, the active components of the photoconductive insulating composition are contained in a single layer composition. This layer is coated on a suitable electrically-conductive support or on a non-conductive support that has been overcoated with an electrically-conductive layer.
Among the many different kinds of photoconductive compositions which may be employed in typical single-active-layer photoconductive elements are inorganic photoconductive materials such as vacuum-deposited selenium, particulate zinc oxide dispersed in a polymeric binder, homogeneous organic photoconductive compositions composed of an organic photoconductor solubilized in a polymeric binder, and the like.
Other especially useful photoconductive insulating compositions which may be employed in a single-active-layer photoconductive element are the high-speed heterogeneous or aggregate photoconductive compositions described in Light U.S. Pat. No. 3,615,414 issued Oct. 26, 1971 and Gramza et al, U.S. Pat. No. 3,732,180 issued May 8, 1973. These aggregate-containing photoconductive compositions have a continuous electrically-insulating polymer phase containing a finely-divided particulate, co-crystalline complex of (i) at least one pyrylium-type dye salt and (ii) at least one polymer having an alkylidene diarylene group in a recurring unit.
In addition to the various single-active-layer photoconductive insulating elements, such as those described above, various multi-active photoconductive insulating elements, that is, elements having more than one active layer, are also well known and, in general, are capable of providing superior performance. In such multi-active elements, at least one of the layers is designed primarily for the photogeneration of charge carriers and at least one other layer is designed primarily for the transportation of these generated charge carriers. Representative examples of patents describing such multi-active photoconductive insulating elements include the following:
Bardeen, U.S. Pat. No. 3,041,166, issued June 26, 1962,
Hoesterey, U.S. Pat. No. 3,165,405, issued Jan. 12, 1965,
Makino, U.S. Pat. No. 3,394,001, issued July 23, 1968,
Makino et al, U.S. Pat. No. 3,679,405, issued July 25, 1972,
Hayaski et al, U.S. Pat. No. 3,725,058, issued Apr. 3, 1973,
Wiedemann, U.S. Pat. No. 3,871,882, issued Mar. 18, 1975,
Regensburger et al, U.S. Pat. No. 3,904,407, issued Sept. 9, 1975,
Wiedemann, U.S. Pat. No. 3,972,717, issued Aug. 3, 1976,
Mey, U.S. Pat. No. 4,108,657, issued Aug. 27, 1978,
Berwick et al, U.S. Pat. No. 4,175,960, issued Nov. 27, 1979.
Smith et al, U.S. Pat. No. 4,282,298, issued Aug. 4, 1981,
Wiedemann, German Patent Application No. 3 019 326, published Dec. 3, 1981,
Graser et al, European Patent Application No. 0 061 088, published Sept. 29, 1982 (corresponding to U.S. Pat. No. 4,517,270, issued May 14, 1985).
Goto et al, U.S. Pat. No. 4,410,615, issued Oct. 18, 1983,
Graser et al, U.S. Pat. No. 4,419,427, issued Dec. 6, 1983, and
Hoffmann et al, U.S. Pat. No. 4,429,029, issued Jan. 31, 1984.
However, multi-active elements of the prior art have typically suffered from one or more disadvantages which have significantly restricted their commercial utilization. For example, they have not exhibited sufficiently high electrophotographic speed, or have lacked a sufficiently broad range of sensitivity, or have been incapable of providing desired contrast characteristics, or have suffered from excessive photoinduced fatigue or from reciprocity failure or from too high a rate of dark decay, or have exhibited excessive electrical noise.
It is toward the objective of overcoming the aforesaid disadvantages of multi-active photoconductive insulating elements that the present invention is directed.